Tuesday, June 07, 2011

Just a quickie….

I got a couple of comments on yesterday’s prop change post, so I thought I’d reply this way, rather than by another comment. Graham on NB Matilda Rose, and Lesley on NB Caxton were both interested in what I’d swapped from and to.

When built Seyella was fitted with a 17 x 12 conventional turbine propeller (the numbers relate to the prop diameter x pitch, which is the theoretical distance in inches the boat should move forward in one revolution. There will be another number as well to identify the size, in our case 1½ indicating a 1½” shaft). This size seems to be the norm for the most common boat size of 57/58 feet.

We’re not really a common boat though. As a liveaboard we’re quite heavy and therefore deeper in the water than would normally be expected for a boat of this size. We draw 28 inches in the summer and maybe an inch or 2 more in the winter (depending how much wood and coal we’ve got aboard!).

That’s stationary, of course. When cruising the stern digs in an inch or two, as well.

So, to cut to the chase, I’ve always thought she was a little underpropped, especially when manoeuvring. And we’ve also had this annoying prop whine at around cruising revs. I changed the prop (like for like) a couple of years ago to no avail, and I also ground “anti-singing edges” on the prop, also with no result.

So I thought, change the size, improve the thrust, and maybe change the harmonics of the drive train. The 17 x 12 was replaced with an 18 x 14 and there’s still plenty of tip clearance under the uxter (recommendation is 10% of prop diameter).

The improvement in power is readily apparent. The noise? Well, I‘m reserving judgement. On a short trip today on the Weaver it was considerably quieter at all revs, so I’m keeping my fingers (and toes!) crossed!

Oh, incidently, Midland Chandlers will take your old prop in part ex. Love 'em or hate 'em, they are useful.

Locks 0, miles 1


Travelling in No Direction said...

Hi Geoff. Do you know the ratio of your gearbox 2:1 or 3:1?

Crew: Mike, Mags, Poppy and Abbey said...

Re: "Whether the different diameter and pitch will change the harmonics of the drive train and get rid of the annoying whine at around 850 rpm remains to be seen."

I had a similar problem, 850rpm (Beta 38)created a metalic whine. Speed up to 900rpm or higher and the noise dissapeared.

It turned out to be an engine mounting rubber that had parted company with one side of the cup. When replaced, the problem has gone. Its a nice quiet burble from the engine.

Geoff and Mags said...

Thanks guys, now that's something I've not checked. Although the noise has "gone away" now, it's still got to be worth a look. Your symptoms sound just like mine...
Regards, Geoff